Distant relation needs to be up, keeping on the ball (5)
I believe the answer is:
alert
'on the ball' is the definition.
(on the ball can mean alert or paying attention)
'distant relation needs to be up' is the wordplay.
'needs' says the answer is hidden in the clue.
'to be up' says the letters should be written backwards (in down clue: letters go upwards).
'trela' is hidden within 'distant relation'.
'trela' written backwards gives 'ALERT'.
'keeping' acts as a link.
(Other definitions for alert that I've seen before include "Vigilant - warning" , "Warning signal" , "Intellectually active" , "Vigilantly attentive" , "alarm" .)